This invention relates to a colorless pressuresensitive copying paper of plain paper transfer type with which a copy or a record can be made on a plain paper.
Hitherto known pressure-sensitive copying papers are products in which an over sheet and an under sheet are superposed so that their coated surfaces confront each other, said over sheet being obtained by dissolving a colorless electron-donating color-forming agent into a nonvolatile oil or the like, preparing a microcapsule by using this "color-forming agent"-containing oil as a core material and coating the microcapsule onto the back side of a support, and said under sheet being obtained by coating a colorless electron-accepting colordeveloping agent onto the front surface of a support. If it is pressed and typed by means of a typewriter or the like, a colored image can be formed on the under sheet. When three or more copies are to be produced, a necessary number of intermediate sheets obtained by coating a color-developing agent and a microcapsule containing a color-forming agent respectively onto the front and back sides of a support are inserted between the over sheet and the under sheet, whereby many sheets of copy can be obtained. Further, there is also the so-called self-contained pressure-sensitive copying paper in which a "color-forming agent"-containing microcapsule and a color-developing agent are coated onto the same surface of a support. They are in extensive use in business paper and the like, because no colored substance is used in them and therefore they do not stain clothes. However, they have a fault that at least two kinds and usually three kinds of coated papers, i.e. over sheet, intermediate sheet and under sheet, must be prepared and the copied image can be obtained only on the intermediate sheet, under sheet or the support of the self-contained pressure-sensitive copying paper which is coated with a color-developing agent.
Further, it is already known that, in the abovementioned self-contained pressure-sensitive copying paper, a colored image is obtainable only on the coated surface and no transfer image is obtained at all on plain paper if the coated surface and a plain paper are superposed and typed.
As used herein, the term "plain paper" means a support on the transfer surface of which neither electrondonating color-forming agent nor electron-accepting color-developing agent exists.
As a pressure-sensitive copying paper having a color-forming agent and a color-developing agent on the same surface of a support and being capable of forming a copied image on plain paper, those disclosed in Japanese Patent Kokai (Laid-Open) No. 126,111/79, Japanese Patent Kokoku (Post-Examination Publication) No. 16,728/78 and Japanese Patent Kokai (Laid-Open) No. 159,008/79 can be referred to.
In Japanese Patent Kokai (Laid-Open) No. 126,111/ 79, it is mentioned to coat, on a support, a coating color obtainable by adding a solid color-forming agent and a color-developing agent into a wax, so as to give a monolayer. It is also clearly mentioned there that said coating color shows a color-forming reaction and is colored just after the color-forming agent and the colordeveloping agent are mixed together. That is, said invention relates to a pressure-sensitive transfer material having a coating layer which is colored similarly to back-carbon paper. It is disadvantageous in appearance because the coating layer is colored. Japanese Patent Kokoku (Post-Examination Publication) No. 16,728/78 relates to chelate color formation. Said invention consists in coating an encapsulated reactant onto a support and additionally coating thereon a co-reactant dissolved in a solvent. Because of the use of solvent the microcapsule wall material is limited to materials resistant to solvent, and there are many disadvantages in the environmental and economic phases.
In Japanese Patent Kokai (Laid-Open) No. 159,008/79, the use of waxes is mentioned. Said invention consists in the so-called two-layer coating in which a microcapsule of "color-forming agent"-containing oil is coated onto a support together with a binder and thereafter a layer comprising a wax and a color-developing agent is coated thereon. In said invention, it is necessary to coat the microcapsule layer on the whole surface with an air knife coater or the like and thereafter to coat the color-developing agent, so that the production involves one additional process which increases the cost. Further, the microcapsule layer is prepared by coating microcapsules onto a support together with a binder in the same manner as in the over sheet of commercial pressure-sensitive copying papers, so that transfer of said layer to plain paper scarcely occurs and the "color-forming agent"-containing oil which has been liberated upon the fracture of capsule occurring at the time of typewriting partially migrates into the support. Thus, the amount of color-forming agent participating in color formation is limited, which is disadvantageous in the phases of transferability and color-formability.
On the other hand, the present inventors have already applied for a patent on a related plain paper transfer type pressure-sensitive copying paper, in Japanese Pat. Application No. 26,390/80 filed on Mar. 3, 1980. Although in this plain paper transfer type pressure-sensitive copying paper a clear image can be formed when a plain paper is superposed thereunder and they are typed with a pressure, a dissymmetrical image is formed on the coated surface of the plain paper transfer type pressure-sensitive copying paper itself or on the surface coming into a direct contact with the plain paper. If this dissymmetrical image remains as it is, it may be usable for the purpose of preventing alternations. However, it is still desirable that color formation does not occur on the coated surface. Said plain paper transfer type pressure-sensitive copying paper has a fault that the color formation of coated surface cannot be prevented perfectly.